Conventional agricultural balers comprise a frame which is travelled on a pair of wheels over a field for picking up therefrom hay, straw or silage grass and feeding such crop material to a baling chamber in which it is compressed to parallelepiped packages under action of a plunger which reciprocates inside the baling chamber. When the packages have reached a predetermined length a tying mechanism is operated to encircle the completed package with a plurality of strands and to knot the strands ends together to form a finished bale which will be ejected out of the baler.
Commonly the baling chamber comprises at least one movable wall section whereof the position can be adjusted to modify the transverse cross section of the baling chamber and thereby vary the resistance met by the reciprocating plunger when a new charge of crop material is introduced into the chamber and thrust against the package. It is well known in the art to equip the movable wall section with actuator means to automatically adjust its position in response to load sensor means sensing the actual force by which the material in the baling chamber is compressed in order to adjust the density of the package to a predetermined value.
Such systems perform satisfactorily as long as new charges are introduced at a regular pace into the baling chamber. However, during normal baling operations, situations may occur where no or hardly any new material is introduced into the baling chamber. This may happen on the headlands, where the baler is turned and where no crop windrows are available, or in field patches where the crop density has fallen very low, e.g. because of crop infection. No material is introduced either when an operating baler is halted in the field. Under such conditions no substantial forces are encountered by the reciprocating plunger and the density control system might derive therefrom that further restriction of the baling chamber cross section is required to obtain the desired bale density. The longer the period no new material is introduced into the baling chamber, the more restricted the cross section of the baling chamber will be. However, at the first full charge of crop material, the restricted chamber may cause such resistance to the movement of the package that the blow of the plunger on this charge effects unacceptable stress on the structure and ruptures part of the baling mechanism. Important damage to the baler may be precluded by providing the drive line of the plunger with shear bolts which break and disconnect the plunger from the tractor drive line whenever such extraordinary forces are encountered. Such shear bolts are satisfactory for dealing with unforeseeable force peaks, but they will only cause frustration when used for coping with the more frequent effects of force amplification on or after turning the baler on headlands.
According to U.S. Pat. No. 4,166,414, issued Sep. 4, 1979 in the name of Garold L. Fleming, et al, the normal operation of the bale density control system may be interrupted by a control valve which is operated by the stuffer mechanism. When the stuffer mechanism does not cycle to introduce new crop material into the baling chamber, the valve precludes that pressurized oil is fed to the hydraulic actuator of the movable wall portion. Such system only works satisfactorily in balers having a stuffer mechanism which is operated in response to a complete filling of the feeder duct. Otherwise the system would not distinguish between conditions wherein new charges are introduced into the baling chambers and others wherein the stuffer mechanism is working idly. It requires a reliable sensor for assessing a complete filling of the duct further in order to preclude inopportune adjustment of the baling chamber.
In the prior art Model D1000 baler, manufactured by the predecessor company of New Holland North America, Inc., of New Holland, Pa., the plunger force was monitored by a pair of load cells which were mounted between the plunger body and the plunger crank arms, as shown in U.S. Pat. No. 4,624,180, issued Nov. 25, 1986 in the name of R. P. Strosser. Adjustment of the baler orifice was precluded when the peak forces of the plunger remained below a threshold level, which was thought to be indicative of an empty running plunger. However it was experienced that the accumulated crop package in the baling chamber expanded slightly after each compression stroke, such that the plunger still encountered a substantial force during the following stroke, although no new material was introduced into the baling chamber. Consequently the threshold level had to be set to a level which was impractically close to the normal bale density settings. When baling loose and resilient crop material such as straw, this system failed to operate reliably.